1. Field of the Invention
The present invention relates to flameholders applied to afterburners and reheaters of jet engines as well as to industrial burner and incinerators. More particularly, the present invention relates to twin-plate flameholder construction which has better performance than that of the conventional ones in terms of the combustion efficiency, flame ignition and blowout limits as well as operation range of the combustion devices.
2. Description of the Related Prior Art
There are three methods commonly used for stabilizing the flame in a combustion chamber of industrial burners, namely: A. using a swirler to produce recirculation; B. using sudden enlarged duct at the joints of two ducts having different dimensions to produce recirculation; and C. using bluff body such as cylinder to produce recirculation. These prior art devices are shown in FIG. 1A.
There are also other methods including the use of a reversing stream and porous plate etc. to create a combustion flow region. Object or objects used to hold the flame stable are all to be referred as `FLAMEHOLDER`.
To meet the requirements of an air-fighter of fast take-off and climbing as well as other tactical operations, usually its turbine engines are equipped with afterburners which enable the engine to boost its thrust force by a large scale within a moment. The afterburner mainly consists of a casing, liner, diffuser, fuel spray bars and flameholders. With this arrangement, the afterburner can boost its thrust force within a moment with a limited weight increase and, therefore, it has been widely adopted by military jet engines. Since the velocity of gases exhausting from the main combustion chamber far exceeds the velocity of spray burning, a flame holding mechanism is required to keep the flame in stable. The flame holding mechanism in these afterburners are a bluff body flameholder, mostly designed as V-gutter. While facing a stream, a recirculation zone is formed due to the blockage and hence the fuel is allowed to extend its residence time at this low speed region so as to hold the flame in stable burning states. This is the major principle of V-gutter flameholder as shown in FIG. 1B.
Since 1950, USA has extensively engaged in the study of the V-gutter flameholder, and until now, all the designs applied to a flameholder in an afterburner of turbine engines are mostly V-gutter in shape. For example, the study made in literature by A. H. Lefebvre in 1988 suggested various kinds of slots formed on a V-gutter construction which concluded as no significant improvement from those designs. On the part of patent priorities, U.S. Pat. No. 2,702,452 issued to TAYLER in February of 1955 and U.S. Pat. No. 3,315,468 issued to VDOVIAK in April of 1967 all adopted V-gutter flameholder as a major construction of its afterburner. In 1973, U.S. Pat. No. 3,747,345 issued to MARKOWSKI in June of 1973 proposed a swirler type afterburner. It was claimed that this design could shorten the length of an afterburner. However, for some reason its actual products did not meet their expectations. U.S. GE Company is in possession of the following two patents; U.S. Pat. No. 3,931,707 issued to VDOVIAK in Jan of 1976 and U.S. Pat. No.3,765,178 issued to HUFNAGEL et al. in October of 1973. These are designed by combining traditional V-gutter flameholder with swirler type afterburner and was firstly applied to J85 turbine engine. In that, it was characterized by having a V-gutter flameholder as main flame holding mechanism with an opening at its leading edge to allow a small amount of air which is guided by swirler to come into flameholder. This design was claimed to have made improvement on flame stability. At a later date, GE applied this design to F404 turbine engine. The TFE 1042 engine developed from a joint-venture between the Chung-Shang Institute of Science and Technology of ROC and US Garrett also adopted the above design. U.S. Pat. No. 4,185,458 issued to ERNST in January of 1980 suggested a flameholder by combining V-gutter flameholder in circumferential and radial directions, and applied to a series of turbine engines manufactured by P&W Company.
There are also other studies made on the flameholder construction such U.S. Pat. No. 3,800,527 issued to MARSHALL et al. in April of 1974; U.S. Pat. No. 4,134,260 issued to LEFEBVRE et al. in January of 1979; U.S. Pat. No. 4,315,401 issued to BEAL et al. in February of 1982; U.S. Pat. No. 4,490,973 issued to KINSEY in January of 1985 and U.S. Pat. No. 4,815,283 issued to ELDREDGE et al in March of 1989. All these studies intended to improve combustion efficiency through modification and additions which merely made its V-gutter flameholder more complicated and its configuration of construction still remained a V-gutter.
The plate type flameholder has been studied in the literature. For example, Gruzdev et al. studied the single-plate type flameholder in 1981. They found that, under the same hydraulic resistance (blockage ratio), it is possible to reduce the weight of the flameholder significantly without degrading the fuel distribution efficiency. These results were attributed to the increase of the burning velocity and blow-out limit.
The idea of using a controllable flat-plate as a flameholder stems from the advantages of its capability to control the flat-plate angle and light weight. This is especially useful in modern afterburner design. The easy control capability allows it to change the blockage ratio as engine thrust rating varies during the flight. The flat plate flameholder can be controlled to the direction parallel to the air-steam during dry power engine operation to reduce pressure loss. The blockage ratio and the inclined angle can be adjusted to meet high power output requirement in normal and combat conditions. The aerodynamic characteristics and the combustion performance of the flat-plate flameholder are hence studied.
Theoretically, recirculation produced from blockage by a bluff body should have a fixed ratio. Based on this principle, inventors proposed in their ROC Pat. No. 56104 an adjustable single plate flameholder in 1992 which proved that an increase in the angle-of-attack (AOA) of single-plate results in the increase of the pressure drop across the single-plate flameholder and increases the combustion efficiency, as well as extending the flame stability limits as compared to those of the conventional V-gutter. Furthermore, this type of flameholder has the advantage of easy control in changing the AOA to satisfy the different operational conditions.
In a later study, inventors further suggested in 1994 a design of a flameholder with a slit stream through the path between two plates to increase the fuel concentration in the wake zone. A modified twin-plate flameholder was hence designed.